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Drum Brakes -- Everything You Need to Know

By Jason Collins | July 21, 2022

What Are Drum Brakes?

Drum brakes are a type of braking system used on many older vehicles and even on the rear wheels of some modern cars. Instead of using a motion of squeezing inward like disc brakes, drum brakes push outward and use that friction to stop the vehicle.

This design is not as popular as it once was since disc brake systems are more efficient and generate less heat. However, rear drum brakes are still used on some new vehicles since they don’t lock up as much and are often used for an emergency brake.

Your braking system is one of the most important parts of your vehicle. Having a high-quality braking system is the best way to ensure that your car will be safe and secure on the road. While it is nice to have acceleration and power, it is much more important that you are able to stop your car on a dime and avoid an accident when needed.

There are two different types of braking systems: disc brakes and drum brakes. Disc brakes are more common in modern cars due to their simple design and easy maintenance but older vehicles and even some newer ones have drum brakes on the rear wheels. Knowing how drum brakes work and how they should be maintained can help you make sure your vehicle is safe.

How Do Drum Brakes Work?

Drum brakes work using pneumatic brake fluid that is pushed into the system when you press on the brake pedal. This fluid pushes a pair of pistons out of the brake cylinder inside the brake drum. Then, the pistons push brake shoes, which are coated with a high-friction material, outward against the drum, which is attached to the wheel itself.

The brake shoes inside the drum are slanted at a slight angle on the friction surface. This makes drum brakes self-actuating, which means they create more brake friction as the brakes are applied. This also means that the shoes need to be pulled away from the drum when the brake pedal is released. Because of this, the shoes are mounted on springs that take the pressure off the drum and let the wheels move freely again.

Difference Between Drum Brake Systems and Disc Brake Systems

When it comes to braking systems, you generally only have two options. These include drum brake systems, which are an older technology and are not as popular as they once were, and disc brake systems, which are much more common in modern, late-model vehicles. While these two systems operate on the same principle of slowing down your car by applying stopping power to the wheels, they do it in two very different ways.

Drum brake systems use friction that is applied outward by the brake shoes on a drum that is attached to the wheel. Disc brakes, however, use brake pads that squeeze a brake rotor attached to the wheel to create friction. Disc brakes have much better braking force since they create more friction. Since the majority of braking power happens at the front wheels, even if your car has drum-type rear brakes, the front brakes are most likely going to be disc brakes.

Drum Brake Components

One of the reasons that drum brakes are not as popular as they once were is the fact that they are very complicated and have a lot of different components. Whenever a piece of machinery has many moving parts, there are more opportunities for one of those parts to fail. Understanding the different components of a brake drum can help you keep your braking system in good working order.

Brake Drum:
The brake drum is a cast iron or aluminum disc that is attached to the wheel hub. This is the component that receives the friction from the shoes and brings the wheel to a stop. They rotate at the same rate as the wheel, so when the shoes apply friction, the wheel slows down and stops the car.

Brake Shoes:
The brake shoes are the components that press outward against the drum when the brake pedal is applied. Brake shoes are curved and have a friction material on the brake lining that helps facilitate the braking power. When this material presses on the drum, it creates the friction needed to stop the vehicle.

Wheel Cylinder:
The wheel cylinder sits on the top of the drum brake assembly and is attached to the brake lines. When you press on the brake pedal, the wheel cylinder pushes out a pair of pistons that then apply force to the shoes. This component applies the force needed to create the right amount of friction.

Retracting Spring:
Because of the design of drum brakes and the fact that the shoes are self-energizing because of the angle they are at, retracting springs are needed to pull the shoes away from the drum. When you take your foot off the pedal, the retracting springs move the shoes and stop the friction on the drum.

Self Adjuster:
When you don’t have your foot on the brake pedal, you don’t want the brake shoes to be touching the drum. If the shoes are resting on the drum, corrosion can happen on the friction area and cause brake fade. The self-adjuster maintains the minimum amount of space between the shoes and drum so they can last longer.

Types of Drum Brakes

Mechanical Drum Brakes

Mechanical drum brakes are used mostly in two-wheeled vehicles such as motorcycles. These types of drum brakes use a brake cam that pushes the brake shoes out into the drum. Because this type uses the direct force you are applying onto the brake mechanism, they are only used in lighter vehicles.

Hydraulic Drum Brakes

Hydraulic drum brakes are the most common type and will be found in older cars and even some modern vehicles. This type uses hydraulic brake fluid to push the brake shoes outward onto the drum. When you press on the pedal, the brake master cylinder creates hydraulic pressure that pushes the fluid through the system.

Pneumatic Assisted Drum Brakes

Pneumatic assisted drum brakes are also sometimes known as air brakes and are used on large vehicles like buses, trucks, and even railroad locomotives. They work similarly to hydraulic brakes but use air instead of brake fluid. This type uses compressed air to turn a cam that slows down the wheel.

Advantages of Drum Brakes

Even though drum brakes aren’t used nearly as often as they used to be, they still offer a few significant advantages. One of the most important benefits of a drum brake system is the fact that they last longer than disc brakes. They have a much larger friction area which creates less deterioration with the shoes. The wheel cylinders in drum brakes are also easier to recondition than the brake calipers in disc brake systems.

Drum brakes are also much cheaper to manufacture than disc brakes. When you couple this with the fact that brake shoes can be reconditioned instead of being disposable, this can translate to a large amount of savings over time. This makes drum brakes a better option for fleets of vehicles or municipal organizations.

Disadvantages of Drum Brakes

Perhaps the biggest disadvantage of a drum brake system is that it can generate quite a lot of heat due to its increased friction surface area. This can cause warping and issues with deterioration within the drum brake mechanism. This excess heat can also create thermal expansion in the drum. When the drum expands, you will need to apply more force to the pedal, which can decrease stopping power.

The other major disadvantage of drum brakes is that they are relatively complex compared to disc brakes. This means that there are more things that can go wrong with them. It also means that working on them and maintaining them is a much more complicated process, which can potentially translate to higher auto repair bills.

Drum Brake Maintenance

Due to their complexity, maintaining your drum brakes is incredibly important. To perform a brake job on your car, jack up the vehicle and remove the wheels. Then, you will open up the drum brake mechanism and look at its condition. There will most likely be some dirt, dust, and debris inside the mechanism and on the backing plate. This can be removed with an aerosol can of air or a compressor.

To look at the condition of the shoes, remove the springs and the shoes will come loose from their retainers. Feel along the edge of the drum and make sure that there aren’t any rough spots that catch your finger. If there are, the drum will need to be reconditioned. Check the recommended thickness of your drum brake shoes in your owner’s manual. If they are more than halfway worn, you will need to replace them.

You should also check for any excess fluid inside the drum brake mechanism. While some moisture is normal, if it is pooling anywhere in the drum, check all your hoses for deterioration and look for leaks in the cylinder. Leaking fluid can cause overheating and a decrease in stopping power with your brakes.

Final Thoughts

Drum brakes may not be as common as they once were but they are still used on the rear wheels and as the parking brake of some new vehicles. Understanding how drum brakes work can help you know what you are dealing with if your car has them and will give you a good idea of how to keep them in working order. By making sure your brake system is in good condition, you can have the confidence you need that your vehicle is always under control on the road.